Glass sheets are conventionally heated to perform bending, tempering, bending and tempering, and pyrolytic filming as well as for other processing. Such heating is conventionally performed in a furnace heating chamber that is heated to a sufficiently high temperature for the particular processing involved and which is maintained at atmospheric pressure. During the heating, the glass sheets can be oriented vertically or horizontally. Tongs are conventionally used in the vertical heating to suspend the glass sheets from their upper edges during conveyance through the heating chamber. Gas support conveyors and roller conveyors are conventionally utilized in the horizontal heating to convey glass sheets through the heating chamber. With the gas support conveyors, gas delivered to the lower surfaces of the glass sheets to provide floating support thereof is supplied from the heating chamber ambient so as to provide heating of the lower surfaces. Both gas support conveyors and roller hearth conveyors have also utilized gas flow directed downwardly against the upper surfaces of the glass sheets to provide heating of the upper surfaces.
The conventional process for tempering glass sheets involves heating a glass sheet within a furnace to a temperature range of about 1100.degree. to 1250.degree. F. and then transferring the heated glass sheet to a quench unit where jets of pressurized quenching gas are impinged against its opposite surfaces. Such quenching rapidly cools the outer surfaces of the glass sheet faster than its center so that upon complete cooling the surfaces are subjected to compressive stresses while the center is tensioned. Due to the compressive stresses at the surfaces, tempered glass sheets are much more resistant to breakage than annealed glass. Also, upon breakage, tempered glass sheets shatter into small relatively dull pieces that are harmless instead of into larger sharp slivers as is the case with annealed glass.
Opposed blastheads such as of the type disclosed by U.S. Pat. No. 3,936,291 are conventionally utilized to supply quenching gas during glass sheet tempering. Such tempering is conventionally performed on flat glass sheets to provide architectural glass, such as in a manner disclosed by U.S. Pat. Nos. 3,806,312, 3,907,132, 3,934,970, 3,947,242, and 3,994,711. In addition, glass sheets are conventionally bent and then tempered between opposed blastheads to provide vehicle glass, such as in the manner disclosed by U.S. Pat. No. 4,282,026.
The U.S. Pat. No. 3,298,810 to Mekelvey, discloses method and apparatus for bending and tempering glass sheets. Nozzles are fed by an air compressor and a gas burner to supply heated gases which supply glass sheets on a curved shaped surface to avoid cross-gauging and to maintain temperature uniformity within the furnace.
Relatively thin glass sheets are much more difficult to temper than thicker ones because the surfaces must be cooled very rapidly in order to set up the thermal gradient between the surfaces and the center. Usually the problem becomes significant when the glass has a thickness of 1/8 inch (i.e. about 3 mm) or less.
The rapid cooling necessary to temper thin glass is conventionally provided by supplying the quenching gas to the opposed blastheads at a much greater pressure than is utilized with thicker glass sheets. Substantial energy is thus required to pressurize the quenching gas in order to temper thin glass, especially at facilities located at high altitudes where the air is much less dense than at lower altitudes. In addition, gas quenching of thin glass is much more noisy than for thicker glass due to the high pressure of the quenching gas used.
The method and apparatus disclosed in the U.S. Pat. No. 4,397,672 to Nitschke is an attempt to overcome the problems associated with tempering of glass sheets especially when thin glass is involved. The quench unit has an enclosed chamber in which gas tempering is performed at superatmospheric pressure. Such a quench unit provides improved tempering as compared to conventional gas tempering due to superior heat transfer that results between the quenching gas and the glass sheet. In addition, tempering of thinner glass sheets is possible and a much quieter operation is involved with the quenching within an enclosed chamber. Furthermore, less breakage occurs when the quenching is performed within an enclosed chamber at superatmospheric pressure.
With the quench unit disclosed in the above-noted Nitschke patent, each heated glass sheet is introduced through an access opening into the enclosed chamber whereupon closing of the opening is followed by pressurization of the chamber to the superatmospheric pressure. During the tempering, energy savings can be achieved if the spent quenching gas within the pressurized quench unit chamber is recirculated rather than being released to the atmosphere. After the tempering is completed, the tempered glass sheet is removed from the quench unit chamber through either the access opening through which it was introduced or through another exit opening. Substantial additional energy savings can also be achieved if the pressurized gas within the quench unit chamber is pumped to a tank before opening of the chamber and then reintroduced back into the chamber from the tank after the chamber is closed for the next cycle. However, a relatively large tank is necessary in order to reuse the pressurized gas within the quench unit and a somewhat complicated fluid handling system is also required to transfer the pressurized gas between the chamber and the tank.